A measure of environmental protection

Peter Stockwell examines the dangers of mercury in industrial coal fired plant emissions and natural gas, and looks at ways in which it can be measured, and ultimately regulated.

The dangers of mercury and its derivatives, especially organomercury compounds have been well documented for almost half a century. Significant concerns remain regarding mercury contamination in aqueous ecosystems. The United States Environmental Protection Agency (US EPA) has advanced water quality criteria for the protection of organisms native to water environments. The chronic water quality criterion for mercury in fresh water ecosystems is 12 nanograms/litre (ng/l), and the mercury chronic criterion for salt water is 25ng/l. These extremely low criteria present significant demands for the currently approved US EPA regulatory methods (1631, 245.1 and SW-846 747770-7471), particularly those which are based on atomic absorption techniques.

We still manage to pollute the atmosphere and environment by the use of coal and petrochemicals. The impact of mercury in natural gas has been such that there have been at least five experiences of plant failure which have been traced to corrosion due to mercury attacks on aluminium rotors in the plant fabric. Burning fossil fuels as a source of electricity also creates a source of mercury pollution. Mercury and oxidised mercury are both generated from the electricity generation, but the fate of these different species is totally different. It is therefore also important to know the speciation profile of the coal-fired stack emissions. The US Environmental Protection Agency is actively pursuing an ambition to regulate the levels of mercury that can be emitted from the chimney stacks, no doubt legislation will follow in Europe. The measurements of mercury in these complex matrices are extremely difficult. Atomic fluorescence provides simplicity, sensitivity and linearity

Atomic fluorescence

The determination of mercury by atomic fluorescence was first reported by Thomson and Reynolds in 1971. Several authors have described enhancements to the technique that have reduced formal instrument detection limits (IDL) for the fluorescence technique to the 1-10ng/l range. Knox et al. report on the use of atomic fluorescence detection limit for mercury to less than 1ng/l. A European standard EN 13506 was published in 2001 and the most recent version of the US EPA standard 1631 utilises an additional gold amalgamation step to lower the limits of detection still further.

Extending the procedures for the measurement of mercury levels in natural gas and coal fired stack emissions, places far more difficult demands on the skills of the analyst. Both present very complex matrices which although different in many respects needs similar requirements to overcome the matrix.

It is important to obtain a sample which is representative of the stream being examined without modifying it in any way. The mercury present should be removed from its complex matrix and transferred into a stream of argon. Passing the sample over a gold substrate which collects all forms of mercury prior to revaporising into a stream of argon has been well documented.

Natural gas is often pressurised above 3000psig and to take a sample at atmospheric pressure it is necessary to design a suitable pressure let down system. For coal fired gas sampling it is important to avoid any condensation of moisture present in the stack gas since with the matrix containing sulphur dioxide, nitrous oxide and carbon dioxide these may result in acidic solutions.

To fulfil the requirements of these measurements PS Analytical has developed the Sir Galahad instrument. This forms the basis of systems which have been developed for fossil fuel analysis. The major problem in any analysis is the sample collection and preparation prior to analysis

Reliable data

Using a pressure let down system which conforms to the European Standard ISO 6978 for natural gas sampling, by careful choice of components and temperature of operation it is possible to provide reliable data on the mercury levels present in the natural gas samples. Online systems can also be devised but these provide additional challenges due to the safety aspects required for process stream measurements at Petrochemical facilities.

Most trace elements released from combustion sources are either non-volatile, or are quickly adsorbed onto particles that are either captured by control devices or deposited locally. Particulate control devices have relatively poor capture of volatile elements such as Hg. Oxidised Hg is water soluble and is generally washed out of the atmosphere or adsorbed onto particles which are deposited locally with precipitation. In contrast to this, elemental Hg has an atmospheric lifetime between six months to two years. Mercury emissions are therefore a global issue since Hg can travel long distances with no distinction of boundaries. One of the most recent estimates for global emissions of Hg from human activities was 5000 tons/year.

Mercury concentration

Current methods for sampling and analysis are known to be problematic and prone to error. In 1996, my research team were invited by the EERC, University of North Dakota to study mercury speciation measurements in flue gas. This proved to be a very difficult application given the low mercury concentration (1-4ppbv) and difficult flue gas matrix.

We developed an online wet chemical speciation module, the hot flue gas was separated into two streams. The first stream converts divalent mercury to elemental mercury. The second channel complexes divalent mercury and the elemental fraction was un-retained. The difference between the two streams is oxidised Hg. The instrument provides the first fully automated online speciated measurements of Mercury in flue gas.

Wherever we look mercury is around us, P S Analytical has shown that it can provide systems fit for purpose and provide reliable analytical data on a continuous basis.

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